The building is located near the intersection of Maroa and Home.
Investigators say the men died from carbon monoxide poisoning and the woman who tried to revive them is still in critical condition.
Darrell Pretzer uses the warehouse to store things for his business. He tells us he let the two men stay here in exchange for some labor. Tonight he's horrified that an act of kindness ended in tragedy.
"I'm just kind of in shock right now,"
Pretzer says he was just trying to help out a friend. Someone he's known for more than 10 years.
"They were both homeless. I gave them a place to stay; they were helping me around the place."
Pretzer tells us he bought a phone card for the men to use and had come to the warehouse to drop it off.
"I went back there and I touched him and shook him and he was stiff, so I knew he was gone."
Darrell says that's when he heard someone breathing. A 9-1-1 operator told him what to do; how to revive the woman lying near the door.
Fire crews say the men had brought a generator inside and used it to power the warehouse.
When emergency crews got on scene it had been running so long it was out of fuel.
Koby Johns of Fresno Fire Department says, "It's ironic two weeks ago we stood here and gave a press conference on the importance of carbon monoxide alarms. We donated some of those to people down the street. It's absolutely tragic something like this has occurred. It's completely preventable."
Johns says last year Fresno firefighters responded to 591 carbon monoxide calls.
"It really hits home how carbon monoxide, how dangerous it is and how it can affect anyone at any time. With a carbon monoxide alarm these men would have been alive."
Friends say they're working to find family members and Darrell says he wishes the two men had followed his advice.
"I told them I did not want that generator inside, I know. I know it's bad."
We're told the woman who was found here inside the home is in critical condition tonight. Firefighters believe she tried to help the men but also fell victim to the carbon monoxide.